The present invention relates to an electric cooking appliance employing heater lamps such as halogen lamps for heating a cooking pan containing foodstuff, and more particularly to such an electric cooking appliance provided with a protective device for protecting the heater lamps against excessive increase of the ambient temperature thereof.
Halogen lamps are generally employed as heater lamps in electric cookers of the type described above which have recently been tried to be made practicable, and heat generated by the halogen lamps is applied to a cooking pan to thereby cook foodstuff contained therein. The halogen heater lamp is superior to a nichrome wire heater in an amount of far infrared radiation to be generated and in that the temperature of the halogen lamp is rapidly raised since the heat capacity of the halogen lamp is small. The electric cooking appliance generally comprises a heating unit including several groups of halogen lamps, each group being made up of a plurality of halogen lamps, and a heat insulator covering the peripheral and bottom sides of the halogen lamps, and a top plate formed from heat-proof glass having a heat transmission property and covering the top surface of the heat insulator, thereby providing for the construction of small heat loss. In use, a cooking pan or the like containing foodstuff is placed on the top plate and the heat generated by the halogen lamps is applied to the cooking pan so that the foodstuff contained therein is cooked.
In the above-described electric cooking appliance, heat generated by the halogen lamps is radiated or transmitted through the top plate to the cooking pan and accordingly the heated object. The heat capacity of the top plate is rendered relatively large and accordingly, the temperature of the pan is not raised rapidly at an initial stage of the heating. The rise characteristic of the cooking pan temperature is lowered. Consequently, the halogen lamps of the high output type (usually 2 kW) have been conventionally employed as the heater lamps.
While, in the electric cooking appliance, the heat insulator and top plate are closely disposed for the purpose of enhancing the heating efficiency such that the interior of the heating unit is rendered a sealed space. For this reason, when the halogen lamps are continuously energized with the output thereof maintained at a high level, the ambient temperature of the halogen lamps or the atmospheric temperature in the heating unit is gradually increased to exceed the heat proof limit temperature (about 850.degree. C.) of a quartz glass tube constituting the bulbs of the halogen lamps or the heat proof limit temperature (about 350.degree. C.) of molybdenum used as material for closing ends of the quartz glass tubes. Continuous use of the halogen lamps at these heat-proof limit temperatures or above quickens turbidity of the quartz glass tubes and oxidation of molybdenum, respectively, thereby shortening the life of each halogen lamp.
In order to solve the above-described problem, the conventional electric cooking appliance is provided with a thermostat as temperature sensing means for sensing the temperature of the interior of the heating unit. When the temperature of the heating unit interior is increased to a value predetermined for protection of the halogen lamps, the thermostat operates to deenergize the halogen lamps. According to this construction, however, when the halogen lamps are re-energized, the temperature of the heating unit interior is immediately raised again with the result that the halogen lamps are deenergized again. Energization and deenergization of the halogen lamps are reiterated at short intervals. Consequently, a problem arises that the lives of the halogen lamps are rendered short. More specifically, since the radiation heat is reflected in the case of a cooking pan of low heat absorptivity such as one with high light reflecting property, heat is liable to remain in the heating unit owing to reflection of heat on the cooking pan. Accordingly, upon energization of the halogen lamps, the heating unit interior temperature is immediately increased to the value at which the thermostat operates to deenergize the halogen lamps and consequently, the halogen lamps are reiteratively energized and deenergized. See FIG. 14. Additionally, even in the case of a cooking pan having dark bottom and therefore high heat absorptivity, the halogen lamps are reiteratively energized and deenergized at short intervals not so much in the cooking pan having low heat absorptivity, as is shown in FIG. 13. FIGS. 13 and 14 each show the relationship between on-off operations of the thermostat and the changes of the surface temperature of an outer tube (which will be described later) of a thermostat heat-sensitive portion and the bulb surface temperature of the halogen lamp.